This invention relates in general to fuel cells, and more particularly to a method and a system for measuring the amount of hydrogen in a hydrogen storage vessel.
In recent years, nearly all electronic devices have been reduced in size and made lightweight, in particular portable electronic devices. This advancement has been made possible, in part, by the development of new battery chemistries such as nickel-metal hydride, lithium ion, zinc-air, and lithium polymer, that enable larger amounts of power to be packaged in a smaller container. These secondary or rechargeable batteries need to be recharged upon depletion of their electrical capacity. This is typically performed by connecting the battery to a battery charger that converts alternating current to a low level direct current of 2-12 volts. The charging cycle typically lasts a minimum of 1-2 hours, and more commonly 4-14 hours. Although the new batteries are a tremendous advancement over the previous generations of batteries, they still suffer from the need for sophisticated charging regimens and the slow charging rates.
Fuel cells are expected to be the next major source of energy for portable electronic products. Simply put, fuel cells catalytically convert a hydrogen molecule to hydrogen ions and electrons, and then extract the electrons through a membrane as electrical power, while oxidizing the hydrogen ions to H2O and extracting the byproduct water. The tremendous advantage of fuel cells is the potential ability to provide significantly larger amounts of power in a small package, as compared to a battery.
Their potential ability to provide long talk-times and standby times in portable communication device applications are driving miniaturization of fuel cell technologies. The Polymer Electrolyte Membrane (PEM) based air-breathing, dead-ended fuel cells are ideally suited for powering portable communication devices. The most mature of the fuel storage technologies for this class of fuel cells is hydride materials packed in a container which stores hydrogen and releases it on demand. Storage of hydrogen in a container containing reversible metal hydride is a common practice in the field of fuel cells.
A fundamental customer requirement for any form of portable power source is the ability to measure and communicate the capacity remaining in the source (how long can it power the device?). In addition, the remaining capacity has to be continually measured while the device is in operation so as to provide the user current status of the power source. In fuel cell systems, as the energy storage and energy conversion aspects are decoupled, the key to measuring remaining capacity depends on the ability to accurately measure the amount of fuel remaining in the storage vessel. In portable devices with fuel cell power sources, the remaining energy capacity is directly dependent on the amount of fuel remaining in the fuel storage vessel. Since many fuel cell applications use hydrogen stored in solid medium such as metal hydrides, chemical hydrides and nano fibers, methods and systems to measure the quantity of hydrogen stored in these media is a necessity for the successful development of these technologies for commercial applications.
Though prior art technologies exist to measure the pressure in a hydride vessel, they do not accurately measure the quantity of hydrogen in the vessel due to the dependence of hydrogen concentration on parameters in addition to the pressure inside the fuel storage vessel. Also, the prior art methods can be used only under equilibrium conditions where the system is not discharging hydrogen to a fuel cell or other load device. Another critical factor to consider is the differences between the charge and discharge characteristics of the storage media. Due to hysteresis, the hydrogen discharge characteristics of the storage medium is different from the charge characteristics. In addition the shortcomings of the prior art techniques described above, they also fail to take into account the degradation of hydrogen storage capacity of the hydride material over time as the percentage of active hydride in the overall composition decreases over time.